(1) Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image on an image carrier such as a photoreceptor, with use of a two-component developer, a process unit having the developing device and the image carrier, and an image forming apparatus having the process unit.
(2) Related Art
There have been image forming apparatuses, such as printers and copiers, that form full-color images with use of the toners of yellow (Y), magenta (M), cyan (C) and black (K), in an electrophotographic method. Such image forming apparatuses are provided with process units for forming toner images of the colors Y, M, C, and K. In general, the toner images of the respective colors formed by the process units are transferred onto an intermediate transfer belt, and thereafter transferred and fixed onto a recording sheet.
Each process unit includes a photosensitive drum that is an image carrier, a charging device, a developing device, and so on. The photosensitive drum rotates around its central axis. The charging device charges the photosensitive drum to form an electrostatic latent image thereon. The developing device develops the electrostatic latent image on the photosensitive drum with use of the toners of different colors. The developing device may develop the electrostatic latent image with use of only the toners of the colors Y, M, C, and K, or with use of a two-component developer including toner and magnetic carriers.
In the case of using the two-component developer, the developing device is provided with a developing roller which includes a housing, a developing sleeve, and a magnet roller. The housing contains toner and magnetic carriers. The developing sleeve has a cylindrical shape, and is arranged facing the photosensitive drum through an opening of the housing. The magnet roller has a columnar shape, and is fixed inside the developing sleeve so as to be coaxial therewith. The magnet roller is provided with a plurality of magnetic poles arranged along a circumferential direction thereof.
The developing sleeve is rotated to develop the electrostatic latent image on the photosensitive drum. As a result, on an outer circumferential surface of the developing sleeve, the carriers form a magnetic brush by a magnetic force of the magnetic poles in the magnet roller. With this magnetic brush, the toner in the housing is transferred. The toner transferred on the outer circumferential surface of the developing sleeve adheres to the electrostatic latent image on the photosensitive drum, at a position closest to the photosensitive drum. In this way, the electrostatic latent image on the photosensitive drum is developed.
Regarding an image forming apparatus including the process units that have the developing devices using the two-component developer, there is a demand for reducing the size of the image forming apparatus and for enhancing the speed thereof. To accommodate this demand, reduction in the size of the process units is currently undertaken. However, when the process units are reduced in size and enhanced in speed, the life of each process unit becomes short due to the deterioration, consumption, etc. of member that constitute the process units. This results in the life of each process unit not corresponding to the life of the image forming apparatus on the whole.
To address this problem, the process units are configured to be replaceable, so that when the process units reach the end of their lives, the process units are replaced with new process units. Each process unit configured to be replaceable is shipped from a factory in a state where a developer is contained in the housing of a developing device. This means that each process unit provided with the developing device using the two-component developer is also shipped in a state where the two-component developer is contained in the housing of the developing device.
In each of the developing devices, the developing sleeve inside the housing is partially caught in and exposed from the opening of the housing. Therefore, when each process unit is transported, the toner in the housing may leak from a gap between the outer circumferential surface of the developing sleeve and the edge of the opening, due to vibration during transportation or the like. When the leakage occurs, the toner inside the developing device is wasted. This impairs cost efficiency.
Also, in a case where an image forming apparatus is transported with the process units mounted therein, the toner contained in the housing of each developing device may leak from the gap between the outer circumferential surface of the developing sleeve and the edge of the opening, due to vibration during transportation or the like. In this case, the toner leaked from the developing devices will stain recording sheets, etc. inside the image forming apparatus.
Patent Literature 1 (Japanese application publication No. 07-20715) discloses a developing device in which a magnet roller is rotated with use of a grip. In this developing device, during transportation, the magnet roller is rotated by 180 degrees from a position where the magnet roller performs operations for image formation after being mounted on the image forming apparatus, so as to prevent toner from leaking from an opening of a housing.
According to Patent Literature 1, the magnet roller in the developing device is positioned such that magnetic poles, which face a photosensitive drum during image formation, face the inner side of the housing during transportation. Therefore, when the developing device is mounted in the image forming apparatus, a user of the image forming apparatus needs to change the positions of the magnetic poles in the magnet roller with use of the grip. However, if the user does not perform operations with use of the grip, the positions of the magnetic poles in the magnet roller do not change, failing in appropriately positioning the magnetic poles. This may cause the electrostatic latent image on the photosensitive drum not to be developed appropriately.
Also, when the magnet roller is rotated by 180 degrees, during transportation, from a position where the magnet roller performs operations for image formation, the toner in the housing is attracted by carriers. In the vicinity of the opening of the housing, however, the toner is not attracted by the carriers. As a result, the toner may leak out during transportation.
To solve this problem, it is possible to include, in the structure disclosed in Patent Literature 1, a device for automatically performing the operations using the grip when the developing device is mounted in the image forming apparatus. However, including such a device may impair cost efficiency, for the device per se has a complex structure, and a complex mechanism is required to provide the device in the structure of Patent Literature 1. Furthermore, it is necessary to secure a space in the image forming apparatus to provide the device, which may cause the image forming apparatus to increase in size.